Many neurological disorders are characterized by abnormal conduction in one or more nerves, leading to unwanted neural activity. In some instances, the abnormal conduction can be associated with pain, spasticity, or other pathological effects realized by different end organs. Examples of neurological disorders can include stroke, brain injury, spinal cord injury (SCI), cerebral palsy (CP), and multiple sclerosis (MS), as well as cancer, joint replacement, endometriosis, hyperhidrosis, vertigo, sialorrhea, torticollis, neuroma, hiccups, and the like. Traditionally, these neurological disorders have been treated using drugs or invasive methods, like neurolysis. Although generally not clinically used, these neurological disorders also can be treated by blocking conduction in the peripheral axons to stop the unwanted neural activity through application of a high frequency alternating current (HFAC) waveform and/or a direct current (DC) waveform.
HFAC waveforms have been shown to provide a localized, immediate, complete, and reversible conduction block without causing electrochemical damage. However, HFAC produces a transient onset response in the nerve, which can take many seconds to diminish and cease. The onset response has not yet been eliminated through modification of the HFAC waveform or electrode design alone. While it is possible to completely neutralize the onset response by applying a brief DC waveform through a flanking electrode, nerve conduction is lost after several applications of the DC waveform. The loss of conduction may be due to the creation of damaging electrochemical reaction products caused by the DC waveform at the levels of charge required to be injected to block conduction in the nerve.
Additionally, DC waveforms can be used as an alternative to HFAC block. Indeed, the DC waveforms can be designed to eliminate the unfavorable onset response or anodic break excitation. However, these DC waveforms can cause electrochemical damage to the nerve. For example, the damage can be due to the formation of damaging electrochemical reaction products, like free radicals, that can be created when the charge injection capacity of the interface is exhausted. The charge injection capacity (or “charge capacity”) generally refers to an amount of charge that can be delivered by the electrode before voltage across the electrode-electrolyte interface leaves the water-window (a voltage in a cyclic voltammogram (CV) between the specific the production of molecular oxygen and molecular hydrogen).